1. Technical Field
This invention relates to thin film capacitors and methods for their manufacture and, in particular, to thin film capacitors in integrated circuits and multichip modules.
2. Art Background
Thin film capacitors are used in a variety of semiconductor integrated circuit devices such as analog circuits, rf circuits, and dynamic random access memories (DRAMs). As observed in U.S. Pat. No. 5,177,670 to Shinohara et al. the dielectric layer in such capacitors should be as thin as possible. Shinohara et al. observes that a good capacitor may not be obtained if the thickness of the dielectric layer is 100 nm or less. One reason for the lower limit on dielectric film thickness is that the breakdown field of capacitors with a thin layer of dielectric material (i.e. the voltage at which the leakage current becomes too high) is typically too low.
Aoyama, T., et al., "Leakage Current Mechanism of Amorphous and Polycrystalline Ta.sub.2 O.sub.5 Films Grown by Chemical Vapor Deposition," J. Electrochem. Soc., Vol. 143, No. 3, pp. 977-983 (March 1996) describes thin (i.e. the thickness of the Ta.sub.2 O.sub.5 film was 85 nm) film capacitors having a breakdown voltage that was less than 2 volts when the gate was positive biased and about 3 volts when the capacitor was negative biased. In Aoyama et al. the capacitor is formed by depositing the Ta.sub.2 O.sub.5 film and oxidizing it to reduce the leakage current through the film. However, to prevent the oxidation of the underlying silicon during the oxidation of the Ta.sub.2 O.sub.5 film, a portion of the silicon electrode underlying the Ta.sub.2 O.sub.5 is nitridized. The resulting dielectric layer is a dual layer of Ta.sub.2 O.sub.5 and silicon nitride (Si.sub.3 N.sub.4) which has an effective dielectric constant that is lower than a single layer of Ta.sub.2 O.sub.5.
Aoyama et al. nitridized the underlying silicon to prevent the oxidation thereof when the Ta.sub.2 O.sub.5 film was oxidized to reduce its leakage. As observed by Kim, I., et al., "Microstructure and Electrical Properties of Tantalum Oxide Thin Film Prepared by Electron Cyclotron Resonance Plasma-Enhanced Chemical Vapor Deposition," Jpn. J. Appl. Phys., Vol. 33:1:12A, pp. 6691-6698 (1994), the effective dielectric constant of the Ta.sub.2 O.sub.5 /SiO.sub.2 layer is lower than the dielectric constant of Ta.sub.2 O.sub.5. The capacitance density of a capacitor is determined by the formula: EQU C/A=.epsilon..sub.0 .epsilon..sub.r /t
wherein C/A is the capacitance density, .epsilon..sub.0 is a permittivity constant, .epsilon..sub.r is the relative dielectric constant of the capacitor's dielectric layer, and t is the thickness of the dielectric layer. Thus, for a given dielectric thickness, a capacitor having a dielectric material with a lower dielectric constant will have a lower capacitance density compared to a capacitor having a dielectric material with a higher dielectric constant. Also, given the inverse relationship between capacitance density and dielectric thickness, the thinner the dielectric material for a certain capacitor, the higher its capacitance density.
Accordingly, capacitors with a layer of high dielectric constant material that is less than 100 nm thick, yet having desirable leakage current and breakdown voltage, are desired.